I don't think I neccessarily understand the issue particularly well, but I've always used knee-over-pedal-spindle (KOPS) as the main rule for fitting myself on bikes (I've read somewhere that it's maybe not as important, but I don't know). I know it feels better for me when I am further forward, so when pedals are at quarter to 3, I still feel like I can put enough power through the front foot (esp important when fixed or climbing seated).
The 56 steamroller has a 73.5 st angle, and the 53 has a 74 st angle, so for the same saddle height, the 53 steamroller would have it's saddle further forward. I think this suits me, as I have small feet and (I think) comparatively short limbs.
As an aside, this is why 'stack and reach' isn't quite the holy grail of bikefit everyone says it is, 2 bikes can have the same reach (distance from bb to stem/bars) but fit differently because of different angles.